Heat oven to 350 F. Coat the inside of a 9 x 9 square cake pan with 1 tablespoon butter. With a mandolin, or a knife, slice the russet potatoes 1/8 inch thick; transfer to a bowl filled with cold water. In a saucepan over medium heat, melt 2 tablespoons of butter; slowly add flour and whisk until smooth. Slowly add milk and whisk until smooth. Cook and stir over medium heat for 3-5 minutes or until thickened. In another saucepan, over medium heat, cook diced white onion in remaining tablespoon of butter for 3 minutes or until tender. Add spinach and cook for 1 minute or until wilted; set aside. Drain potatoes. Arrange a single layer of overlapping potatoes to cover the bottom of the cake pan. Pour 1/2 cup of milk mixture over potatoes. Spread with 1/2 cup cheese, 4 slices of ham, and 1/2 cup of spinach and onion mixture. Repeat layering. Cover with aluminum foil and bake for 20 minutes. Remove foil and top with remaining cup of cheddar cheese. Bake an additional 5 minutes to melt cheese; remove from oven and let sit for 5 minutes before serving. Serves 7.